Frequently asked questions

The third Krona app, with its own phone app, watch app and watch face: the next 24 hours of the sky on one ring — when the Sun and the Moon rise and set, where the planets stand, which satellites pass — and whether the clouds will let you see them. Rise, set, phase, eclipse and pass times are computed on your device.

Neither. Krona Moonrise is free and complete without one: the ring, the satellite passes, the compass and the sky view, the cloud along the line of sight, the star glow, the aurora, the widgets, the watch app and its face. Premium Weather is optional. Krona Moonrise and Krona Sunlight are two separate apps; neither needs the other, and Sunlight's Premium does not include Premium Weather.

• Rain, dew and frost as a line between the Sun's and the Moon's arcs
• Minute-by-minute rain from radar and satellite, up to four hours ahead, in that line
• The weather at each satellite pass: dew or rain, and the air temperature
• Haze from the Copernicus aerosol forecast in the star glow

$0.99 a month or $9.49 a year, with a 7-day free trial. Subscribe once in the phone app; the watch unlocks with it. Cancel anytime on Google Play — you keep Premium Weather until the period ends.

As good as the finest weather model that covers you:

• Finest, for the next two days: Germany and its neighbors, from Britain and most of France to Denmark, Czechia and northern Italy (DWD ICON-D2, 2.1 km); the contiguous United States, southern Canada and northern Mexico (NOAA HRRR, 3 km). There the first 18 hours come from runs made every hour.
• Good: the rest of Europe, from Iceland and Portugal to Moscow and Turkey, and the Mediterranean's southern shore (DWD ICON-EU, 6.5 km) — also the third day around Germany.
• Everywhere else: the global model (DWD ICON, 13 km), which tells a cloud deck over your region from a clear night but cannot place a single cloud — also the third day in North America.

The older the forecast for an hour, the broader its shading, in place and in time: an hour from this hour's run is drawn as sharp as the model allows, the same hour from a run made yesterday is spread over tens of kilometers and a few hours. A forecast days out cannot place a cloud to the kilometer or to the hour, and the app does not pretend it can.

As fresh as the weather models make it. Over central Europe and North America the finest models run every hour, and those runs cover the next 18 hours; the runs that reach further come every three hours over central Europe and every six elsewhere. Our server takes each run in within half an hour of its release, and the app checks for a new one every half hour, fetching the hourly runs and the longer ones apart, so most updates are the next 24 hours alone. The satellites' orbits are refreshed twice a day and the aurora forecast every half hour.

A forecast says how much of the sky is covered. Krona Moonrise follows the line from you to the Sun, the Moon or a planet up through the low, middle and high cloud, where that line actually crosses each layer — a low Moon behind a cloud bank to the east is dimmed while the sky overhead is clear — and weighs how thick the cloud is: a veil of cirrus dims a little, a thick deck hides the body.

How deep into the stars you could see, hour by hour, straight overhead: brightest on a dark, clear night. It weighs how dark the sky is — twilight, and the Moon by its phase and height — the haze in the air and the cloud; bright moonlight dims it and overcast puts it out. It knows nothing of streetlights, so it describes the natural sky: from a city you will see fewer stars. You can switch it off under Layout.

On the satellite-pass and compass pages, the button in the corner is a live compass needle that points north. Tap it and the page shows the sky the way the phone points: the ground and the horizon with its compass points, and the Sun, the Moon, the chosen planet, their paths and the satellite's pass where they stand. Hold the phone above your head and the compass turns over, to be read from below. It uses the phone's compass and motion sensors, which need no permission, and nothing about it leaves the phone. On the phone only.

The ISS, China's Tiangong, Hubble and four more, computed on your phone from their orbital elements, which our server fetches from CelesTrak. By default only the passes you can actually see: the satellite lit by the Sun while your sky is dark.

When the Kp forecast from NOAA's Space Weather Prediction Center brings the northern or southern lights into sight where you are, in the dark, the aurora joins the Planets & more list — and chooses itself when the cloud forecast says the sky toward it is clear. Its band shows how high above the poleward horizon the lights would stand. It is a forecast of geomagnetic activity, not a promise of a display.

Yes. At the end of the Planets & more list, Custom… searches a catalog of about 85,000 stars, 12,000 galaxies, nebulae and clusters, Pluto and the other dwarf planets, 26,000 named asteroids and 1,000 comets — by name in five languages, by nickname or by catalog number (M31, NGC 7000, HIP 32349, α CMa, 1P). Keep up to ten; one at a time takes the planets’ place on the ring, in pink with a symbol for its kind, on the phone, the watch, the widgets and the face. Stars and deep-sky objects are computed on your phone; a dwarf planet, an asteroid or a comet takes a four-day position forecast from NASA JPL that our server fetches. A search sends only what you type. Like the planets, these objects set off no alerts.

Under Layout, and on the tour’s first card, you choose what you look with: the eye, binoculars (10×50) or a telescope (150 mm). Every visibility in the app follows it: whether a planet, the Moon, Apophis or one of your objects can be seen, the star glow and its words, the satellite passes (with binoculars at most — a telescope cannot follow a pass) and the alerts. A galaxy or a nebula is judged by its brightness spread over its size, which an instrument enlarges but does not brighten. Meteors and the aurora are counted by eye.